A noun clause does the work of a noun (or a noun phrase). A noun clause can be object of a verb, subject of a verb, object of a preposition, apposition to a noun, apposition to “it”, and complement of predicative adjectives.
My wife said (that) she was pleased to welcome my Romanian friends.
The noun clause in italics refers to the verb “said” – it is the object of the verb “said”.
An noun clause that is the object of a verb can be a statement (as in above example) or a question (He said, “Where do you live?”; He asked me where I lived.; Can you tell me where you live?)
The noun clause that is subject of a verb is always placed before the principal clause. Please notice the difference between object of a verb and subject of a verb.
What I am writing here seems very difficult for learners of English.
Question: What seems very difficult?
Answer: What I am writing here.
The clause in italics is the subject of the verb “seems”)
My Vietnamese students will be thankful for whatever I teach them.
As you can see the clause in italics refers to the preposition “for”. They are thankful for ...
The news that we are having two days off is not true.
The news is not true. What news? We are having two days off.
The clause in italics creates a relation between the two expressions: “The news …” and “… is not true”.
It is certain that he missed the train.
The real subject here is “that he missed the train”.
The clause in italics in this example is quite similar to the following one. “It” is the formal subject but in this situation the real subject is the noun clause.
I am certain that he missed the train.
Please notice the difference between this example and the previous one.